The present invention relates to a semiconductor laser printer, and more particularly to the processing of a binary video signal carrying information of dots to be printed.
In the conventional printer, a binary video signal for "printing" one dot on a light sensitive medium or for irradiation of the light sensitive medium with a laser beam for one dot includes a unit portion which has a relatively higher lever for the whole of a bit time or a duty factor of 100%, as has been disclosed by, for example, JP-B-62-59508 (corresponding to JP-A-61-234168 laid open on Oct. 18, 1986). Accordingly, in the case where consecutive dots are to be printed in a main scanning direction, a binary video signal supplied from an information processing apparatus to the printer are formed by consecutive unit portions each having a duty factor of 100% or a relatively higher level is continuously maintained. Therefore, if the spot of a laser beam has not a longitudinally elongated shape with the dimension of the spot in a lateral direction (or scanning direction) larger than a half of the pitch of dots to be printed, there is a tendency that the contrast potential (or sensitivity) of the light sensitive medium or drum upon beam irradiation for printing of one dot becomes low as compared with that of the light sensitive medium upon beam irradiation for continuous printing of two or more dots with a result that the one dot is lightly printed. In order to suppress this tendency, the conventional printer employs a method in which an isolated dot is detected to increase a current to be supplied to a laser oscillator, thereby improving the quality of image or print.
However, the correction method in which the current of the laser oscillator is instantaneously increased has the drawbacks in that the number of highspeed constant current switching circuits is doubled, that work for adjustment of fluctuations of the slope efficiency of the laser oscillator is correspondingly doubled, and that a laser output power is liable to fluctuate owing to fluctuations of temperatures and/or adjustments of two constant current switching circuits, since the output power of the laser oscillator (especially, a semiconductor laser oscillator having a high sensitivity) encounters a large change for a minute change of a photocurrent in the laser oscillator.
FIG. 1 shows, as to the conventional printer, a relationship between a binary video signal (Video Data) including a signal part for one dot and a signal part for three consecutive dots, the scanning of a laser beam spot 1 (in an x-direction), and light energy distributions (2, 3 and 4) on a light sensitive recording medium or drum. In this conventional printer, the longitudinal and lateral dimensions x.sub.1 and y.sub.1 of one dot "printed" on the light sensitive recording medium are different from each other or the dot formed has not a substantially circular shape, as is apparent from FIG. 1. In the figure, x.sub.2 denotes the lateral dimension for the three consecutive dots. E.sub.A and E.sub.B represent beam spot energies, and E.sub.thx and E.sub.thy represent the energy thresholds of the light sensitive recording medium. E.sub.AP1 is a peak value of the energy distribution curves 2 and 3.
According to one method of compensating for the different longitudinal and lateral dimensions of one dot, the duty factor of a signal part for one dot is made smaller than that of a signal part for two or more dots to satisfy x'.sub.1 =y.sub.1, as shown in FIG. 2. However, in this method, a light energy distribution on the light sensitive recording medium is changed from the light energy distribution 2 in FIG. 1 to a light energy distribution 5 in FIG. 2. Therefore, the conventional printer employs a method in which a laser driving current is increased for only a signal part for one dot to obtain a light energy distribution 6. In FIG. 2, E.sub.AP2 represents a peak value of the energy distribution curve 5.